1. Field of the Invention
The present invention relates to a method for laser crystallization of an amorphous silicon thin film for forming a thin-film transistor or the like.
2. Description of Related Art
Conventionally, thin-film transistors that are applied to a variety of technical fields of TFT-LCDs, image sensors, touch-panel sensors, and the like are formed in an amorphous silicon thin film that is formed on a substrate; recently, however, various techniques are employed to crystallize the amorphous silicon thin film for the enhancement of response rate. The most widely used crystallization method is a laser annealing method in which an amorphous silicon thin film is irradiated with an excimer laser beam having a wavelength of 308 nm or a second harmonic YAG laser beam having a wavelength of 532 nm to crystallize the entire surface of the amorphous silicon thin film.
FIG. 5 is an explanatory drawing showing a typical conventional laser crystallization method. As shown in FIG. 5, the method of laser beam irradiation is such that the line beam L of an excimer laser is formed by constructing an optical system using an folding mirror 10, slits 11 and 12, an annealer window 13, and like components, and the substantially entire surface of an amorphous silicon thin film 15 that is formed on a substrate 14 is irradiated as the beam travels in the direction of the arrow X (scanning irradiation). Excimer laser annealing apparatuses that give a line beam (e.g., the line beam L) with a beam width (line beam width LW) of 465 mm are produced in a commercial scale. Further, an irradiation technique with which the entire target surface is irradiated with a planar laser beam at once has also been researched and developed.
In addition to the method that uses a laser of this type, Japanese Unexamined Patent Publication No. 2004-134577, for example, discloses a laser crystallization method in which a laser beam absorption layer that is made from an alloy containing Cr, Mo, or the like, or that has a layered structure thereof is formed on an amorphous silicon thin film by vacuum deposition or sputtering, and the absorption layer is then irradiated with a semiconductor laser beam for heating and crystallizing the amorphous silicon thin film.
However, the conventional laser crystallization methods are problematic in that since the substantially entire surface of an amorphous silicon thin film is irradiated with a laser beam in the form of a line beam as the beam travels across the surface, heat generated by the beam creates stress in the glass or silicon substrate, resulting in cracking or warping.
To alleviate the thermal effect on the substrate, it is effective to crystallize only the specific areas of an amorphous silicon thin film necessary for forming a thin-film transistor or a like device; however, it is difficult in a method in which the entire surface is irradiated with a line beam of a laser, such as an excimer laser, to crystallize only the specific areas of the amorphous silicon thin film because the surface of the amorphous silicon thin film is entirely crystallized (polysilicon formation) and then patterning is performed by exposure/development according to a photolithographic method.